When high-maintenance superhero divas get together, things don't always go smoothly. Even for a hard-ass like Nick Fury (Samuel L. Jackson), head of the ultra-secret S.H.I.E.L.D agency, getting a billionaire genius in a metal suit, a scientist with a bad temper, a cryogenically preserved WWII warrior, a Norse god, and two secret agents (how did they get in?) to play nicely together can take a while. For Joss Whedon, it takes around two hours, and even then the film turns into a hipper version of Transformers 3.

Give Whedon credit — he's able to use brisk montages and/or dialogue to provide the basic back-stories of Iron Man/ Tony Stark (Robert Downey Jr.); The Hulk/ Bruce Banner (Mark Ruffalo); Captain America (Chris Evans); Thor (Chris Hemsworth); Hawkeye (Jeremy Renner), and the Black Widow (Scarlett Johansson). That doesn't give him a lot of time to smooth out the rough edges, especially when he also must explain how Thor's bitter sibling Loki (Tom Hiddleston) wants to conquer the world by sending in a demon army through a Stargate-like portal.

So Whedon must plod through a lot of exposition, bickering, and bad dialogue, in which everyone's favorite heroes come off not quite as good as they would be in a film by themselves. The top-of-the-line cast, for example, doesn't always have a lot to work with. Renner's Hawkeye turns into a zombified minion of Loki at the very beginning. Jackson seems mildly distracted. Ruffalo, who spends most of the film in stuffy Bruce Banner mode, applies the Method approach to his role and looks like someone trying to remember where his keys are. Loki, on the other hand, doesn't exactly chill the blood: he looks like Woody Allen in his jester garbfrom Everything You Always Wanted to Know About Sex. And his minions aren't very daunting — even mere mortals like Hawkeye and the Black Widow swat them away like pesky houseflies.

But Downey retains the sardonic wit that put the irony in Iron Man, and when Whedon taps into that antic spirit — like the shot of the Hulk delivering a sucker punch that got the biggest laugh at my screening — the film finds its groove. And despite the murk and unwieldiness of 3D, Whedon orchestrates action sequences that are elegant and exciting, like a long, sweeping shot across the ruins of Manhattan from one embattled Avenger to the next that deserves the term "epic."

So maybe the awkward first steps are over and the franchise will gain traction in the sequels. More topical references might help. Here, the feeble 9/11 allusions and generic paranoid conspiracies go nowhere. But what the Avengers really need is something to avenge, some compelling emotion or commitment; rallying the troops with blood-stained collectible trading cards doesn't cut it.